Protection of metals against corrosion



March 30 1926.

l. T. BENNETT PROTECTION 0F METALS AGAINST CORROSIN Filed J 26 1924 iSnom/ttor /rU/ng 7156/7/76# 3g) fbi@ w 47 LW Passata M". 3o,` 192e.

'UNITED STATES PATENT'oFFlca.

IR'VING T. BENNETT, 0l' BROOKLYN, NEW YORK, ASSIGNO TO THOHAS E.IUR'RAY',-

Ol' BROOKLYN', NEW YORK.

PROTECTION OF KETALS AGAINST COBBOSION.

Application Medlime 26, 1924. Serial lo. 788,462. j

To all whom it may concern.' v

Be it known that I, IRVINe T. BENNETT, acitizen of the United States,and a resident of Brooklyn, county of Kings, "and State of New York,have invented certain new and useful Improvements in the Protection ofMetals Against Corrosion, of which the following is a specification.

When steel and other metals are subjected to high temperatures the tendto oxidize more rapidly than at or inary atmosheric temperatures.Various methods have geen devised for treating steel to increase itsdurabilit under such conditions. The most successfu of such methods haveinvolved the formation upon the surface of the steel of a continuous,adherent film of oxide, which is non-porousand of low ox gen solubility.'This film thus protects t e underlying metal from further oxidation.

In practice I have found that the best of the known methods is effectiveonly at moderately high temperatures, and that at exceedingly hightemperatures the surface protection isl not durable. My presentinvention aims to provide a surface rwhich shall be durable atvexceedingly high temperatures and which can be economically ap lied tosteel, or iron, or similar metals; suc as, for examine, low carbonsteel. I have demonstrate' its success particularly with boiler tubesmade of steel containing 0.15 to 0.20 per cent carbon. The invention isof particular Ivalue in connection with tubes provided with fins, asdescribed in application No. 715,369, andY other applications, of ThomasE. Murra These tubes have longitudinal fins wel ed on to them and areinv some cases located in the boiler furnace and exposed to the radiantheat of the burning fuel, so that they maybe heatf ed to an exceedinglyhigh temperature, appreaching 2500 degrees By treating such ns inaccordance with the present invention, their durability under suchconditions is eatly.- increased.

he articles of steel protected by the process are adapted also tovarious other uses, havin a surface which is practically non-corrodl leunder ordinary conditions. The treatment ma in fact, be so ap lied as tomake the articls practically inso uble in all acids, exce t forvv'e slowsolubility in aqua regia an hydroch orio acid. The acof chromium 2. Therod thus produced is then electroplated to provide a second coating 3 ofnickel.

The article with these coatings deposited thereon, is then heated fortwo hours or' more to a temperature in the range 1800 de! grecs F. to2500 degrees F. in an oxidizing atmosphere. In this heating operationthere are formed apparently alloys as indicated in Fig. 2. The outersurface is covered by a very thin continuous film ofextremely adherentoxide indicated by the heav line 3, which has practically the same coecient of expansion as steel, and which is very dense or non-porous. Thisisy probably a mixture of nickel oxide and chromium oxide intimatelydispersed and of low oxygen solubility.v I mmediatel coating 4, in allproba ility an alloy composed chiefly of nickel and chromium, in solidsolution, which adheres closely to an inner coating' probably composedchiefly of iron and chromium in solid solution; which in turn adheresclosely to the steel 1. After heating it will be noted that there aretwo different layers. But these are not of the same composition asbefore heating. The' outer layer 4 has a hi h resistance to acids and toextensive oxidation; whence, and from its appearance, I conclude that itis an alloy of nickel and chromium in solid solution similar to knownalloys of these two metals. f

The inner coating 5 merges `into the steel in an ill-defined line. Insomeplaces the microsco indicates the formation of pearlite litween thisinner coatin steel `foundation. The coating 5 1s apparently an valloycomposed chiefly of iron and chromium in solid solution, bein hard toetch and resistant to' corrosion. hromium below` this is theprotected inaccordance with the of carbon, is electroplated to form a coating andthe e Yiis iron into each other.

in this inner coating is also indicated-byk the ,formation of pearlitereferred to, it being known that chromium lowers the eutectoidcomposition and permits the formation of more pearlite than in the caseof ordinary carbon steels of a similar carbon content. Y

Chromium and iron alloy in th solid state when heated together under pror conditions of contact and atmosphere. ut unless the conditions arereducing, the clito-- mium oxidizes and union with iron does not take.place. In my process the outer layer of nickelprotects -the chromium fromcxidation during the .heatingi operation and gives it achance"topenetrate the steel and alloy with it.n At the same time thechromium alloys with the nickel toform a heatresistant alloy whichtogether with the thin 'lm ofadherent oxide aforementioned furtherprotects the chromium -iron alloy, which directly' protects the steel.It is probable that migration of the chromium and nickel into each otheris more rapid than the migration of the chromium andV Thus the chromiumcoating is .promplgr vprotected and time iS givenfoi'` the for ation ofthe iron chromium alloy.

The steel. with the chromium and the nickel coating deposited thereonmay be considered lthe finished product, ready to beput into use whereit will be subjected to high temperature. In that case` the conditionsof use: will lix the coatings bythe oxidizing and lalloying eiects whichhave vbeen referred to.

Or the article coated withchromium and nickel ma be heated to fix thecoatings as described fore itis put into use; In eati ing the coatedarticles it is Iireferable to luse an oxidizing atmosphere for, aspreviously mentioned, the success of the process depends on the'formation of a pro r oxidev coatin together with alloying e ectsproduced y the heating.

Chromium is preferred because -it is highlly resistant to heat andbecause it combines readily with iron to-'formuhigbl non-corrosivealloys, some common y own.- as stainless steel. Nickel is preferred forthe outer. coating because it readily combines with chromium to formhighly resistant ale 4invention may be applied,l however,- with othermaterials than chromium land nickel. Also, though I have foundvelectroplating a' convenient method of applying the metals, yet the .maybe applied in other known or-suitab el ways.

The process is not confined to a single coating of each metal.' There'ma be several. coats of each metal, the two ing-deposited alternately;and indeed other metalsmay be added. As an example of the method ofdepositving the chromium, a bath the followingcomp'osition:

Chromic acid, 200-250 grams; chromium sulphate, *4 grams; water, 1000cc.; anode wof chromium; cathode'of steel; cathodecur-` rent density persq. ft. 100 amps.' solution E. M. volta f I have found that two hoursAbath willl give a satisfactory deposit, and

that agitation will aid this deposition.

in such a4 used may be of Y .An example of the 'method of depositing1it)heh:nckel involves the use ofthe following Boric acid, l5 gm.;nickel-ammonium sul phate, gm.; ammonium chloride, 15 gin.; water, 1000cc.; anode of nickel; cathode.

Lof chromium deposited on steel; cathode current density (amps. per sq.ft), 2-3; so-

lutioii E. M. F., 1.5,volts.

Two hours in this bath givesa satisfactorydeposit.

In usinga greater number of coatings I propose, afterthe firstcompoundcoating of chromium and nickel, to coat in succession withchromium, iron, chromium and nickel, repeating thisseriesa'sdesired;the`

purpose of the iron being to alloy with the chromium on either side ofit so that the heated article will consist of several layers i of thetwo different alloys.4 Though the above order of deposition of thevarious metals is preferred,'any other order which gives satisfactoryresults maybe used; that is, the nickel may be deposited on the iron andthe chromium on the nickel, with` but sli ht dierence in properties.

have subjected test pieces made as above with asingle coating of thechromium and nickel to temperatures of 1800 and 2200 degrees F. vfor twohours in an oxidizing-atmosphere and found the article to maintain agood looking surface withh oxidation to a depth of only about oneten-thousandth of an inch; whereas the same steel, untreated, subjectedto the s'ame testwas oxidized to a depth of from three hundredths toseven hundredths .of an inchv and badly scaled. With a temperature ofabout 2500 degrees F.

for half an hour ordinary steel was badly derlying steel. Ihave'subjected test pieces madeas above with a sing e coating of thechromium 'and nickel to -ter'nperatures of,

1800 to 2300 degrees F.' for 576 hours in an f oxidizing atmosphere landfound the article to maintain a' d looking surfa with 'ox-- idation to adepth of only one ten-thousandth of an inch; whereas the same steeluntreated subjected to the same test was entirely destroyed rbyoxidation2 burning u completely. Articles coated with chromiui y isccorrosion-resisting and acid-resisting ro crties of the coated steel tobe very high Taking a lsample which had been electroplated as abovedescribed and then. heated in an oxidizing atmosphere to cause theoxidizing and alloying action, I immersed it in various acids andcompared the results with those obtained by similar tests oncoininercial acid-resisting metals, chromium,

nickel and alloys of nickel and chromiumy and ofl chromium and iron.Steel treated in accordance with the invention and heated to 1832degrees F. for two hours showed a resistance to corrosion equal to thatof chromium in -all acids except concentrated hydrochloric acid in whichit far surpassed chromium, being practically insoluble while thechromium rapidly dissolved. In fact, the protective coating on the steeltreated according to` this. invention may be made to approach platinumin resistin acids. .This resistance is due, in all robabi ity, chieflyto insolubility of the oxi efilm'and, secondly,

to the two alloys underlying this film which are in themselves verysimilar to commercial resistant alloys.

l The' corrosion resisting pro rties of the treated vsteel make it.valuab e in laboratories and workshops where strong acids are ypracticall or `substantiallyl its entire employed or where, becauseofthe corrosive fumes presentfit would be impossible to use untreatedsteel.

The invention may be applied also to jthe protection of a. piece oflmetalmadeup1 vk t ic ' ness by t is process; that is, starting withvthickness of the a small core and building up with successive coatingsto the desired size. For example,

the fins 8 of Fig. 3 might be madeup".-

throughout all or the greater partof their successive coatings-diescribed above. 1

I have stated above my theory as towhattakesplace in theV application ofmy process'. And it seems clear that the process is de-v pendent on thedidusion of metals in the solid state, resulting in the formation ofalloys or series of alloys which are extremelv ture applied and thelength of time of its application, and the product may vary in thisrespect within a considerable range.

for,

Such a variation is'to be expected-,also where the metals used are ofdifferent compositions. For example, I have referred to steel or iron orsimilar metals, which I may include in the term ferrous metals, andIhav'e got-ten satisfactory protective results with both cast iron andwrought iron.- But I should expect the direct results of the treatment1to be different for each of these base` meta s.

The invention contemplates not only the use of nickel but also the usein the sameA way 'i'or of equivalent metals; that is, metals closelyallied in their chemical and physical properties, belonging to the samechemical grouD and occupying similar places in Mendele jcis periodictable.

Though I have described with great particula-rity of detail certainembodiments of my invention, it is not to be understood therefrom, thatthe invention is restricted to the embodiments illustrated. Variousmoditications thereof maybe made by those. skilled in the art withoutdeparting from the invention as defined in the following v claims.

What I claim is: l. The method of protecting the surface of ferrousmetal, which consists in applying chromium thereto and applying aprotective i layer over the chromium.

2. In the protecting of the surface of fertous metal, the method whichconsists `in ap'- plying chromium thereto and applying nickel overthechromium.

3. The method of protecting the surface of ferrous metal, which consistsin applying chromium thereto and applying to the surface an adherentoxidecoating.

4. The method of protecting thesurface of ferrous metal, which consistsin applying chromium thereto' and applying a protective layer over. thechromium, and heating the article to` cause 'thealloying of the ironandr chromium, and to` form on the outside an' adherent oxide coating; l

j 5 The vmethod of protecting the surface of ferrous metal, whichconsists in applying chromium thereto and .ap 'lying nickel overtlie'chromium, and heating the article to cause the alloying of the ironand chromium and of the chromium and nickel, and to form on the outsidean adherent oxide coating.

f A,6. The method of protecting the surface l of ferrous metal, whichconsists in applying thereto electrolytically .a coating of chroinium,protecting the latter by an outer coating, and heating Athe article tocause the chromium to alloy'with the iron, and to c form on4 the outsidresistant to oxidation and yto. acids. .But

such diffusion is dependent on the tempera# e anl adherent oxidecoating;

7. T e method of protecting the surface of ferrous metal', whichconsists in applying thereto electrolytically a coating of chromiuin,applying an outer coating of nickel to 'thachfomiummdvhming the miele ucause the chromium-to alloy with the iron and the nickel to alloy withthel chromium,

` :fand4 to form on the 'outsidef'an adherent plying a' coating ofchromium to ferrous metal and 'a coating of nickel to the chrog 9.' Themethod of producing; composite.

piece of metal which consists in applymn coating of chromium to ferrousan navega'.

coating of nickel to the chromiu'm'and heat ing the same to cause analloying of the iron la and chromium `und of .the y chromium and Anickel and to cause the formation of anou`t sidevcoeting of adherentloxide.

10. The method of rotecting the of ferrous metal whic "consists'inapplying 2 thereto rotective la .one of chromium and anotlher of nickel.

In witness whereof, I have hereunto signed 'IRVING pr;-lim'nifr'r.r

